The present invention is related to a piezoelectric ceramic transformer and, more particularly to a structure for the above-type transformer which provides multi-mode adjustments for various outputs and a high power efficiency.
Piezoelectric ceramic transformers are small in volume, simple in construction, light in weight and have high transforming ratios which are advantageous over the conventional transformers including a magnetic core and windings. The monolithic piezoelectric ceramic transformers, however, have limitations in the designs of topography and polarization, and the majority of the transformers only comprise one operation mode (.lambda. mode or .lambda./2 mode) such that the transformers are polarized with difficultly, have lower operational frequency and can generate a large amount of noise in operation.
One of the conventional piezoelectric transformers is disclosed in U.S. Pat. No. 3,736,446 by Berlincourt et al. The disclosure teaches a ceramic piezoelectric transformer made of ferro-dielectric material which comprises a third set of electrodes coupled to one of the other sets of electrodes enabling the transformer to accomplish self-oscillation. The substrate of the transformer has different polarization directions to provide resonant voltage output. Although such a transformer has a .lambda./2 and a .lambda. operation modes simultaneously (.lambda.=the wavelength measured in the direction of the wave propagation), it can not efficiently enable or disable any one of the modes as desired. Further, the transformer only has one load voltage output terminal so that the transformed ratio of the voltage or current will be low due to the limitations of the length, width and thickness of the piezoelectric substrate thereof. Besides, for a large piezoelectric substrate, the polarization thereof is difficult.
U.S. Pat. No. 3,659,127 discloses a piezoelectric ceramic transformer with specific width to length ratios of which the piezoelectric substrate is composed of an input portion and an output portion. The patent comprises first and second vibration modes (.lambda./2 mode and .lambda. mode) and the output voltages thereof are adjustable by changing the ratios of the length to width of the piezoelectric substrate. In this case, to obtain higher voltage outputs, the length and the width of the substrate should be increased which will result in difficulty in polarization of the substrate. Further, the transformer has only one load voltage output so that it can not enable or disable any of the vibration modes efficiently.
U.S. Pat. No. 2,830,274 teaches a piezoelectric transformer of which the central electrode is polarized transversely to the plane of the substrate of transformer and the remaining portions (the portions to the right and left of the central portion) are polarized longitudinally in a single direction. The central portion further comprises two short-circuit circular electrodes. Transformed voltages of the two circular electrodes are output from two terminals of the substrate. Since the polarization direction of the right and left portion in this case is consistent, the desired polarized voltage will be larger and the polarization process of the piezoelectric substrate will be more complex in comparison with the present invention having a substrate with the same size. Besides, the patent only has a .lambda./2 operation mode and thus, the transformer will have lower operational frequency resulting in a large amount of noise.
Another prior art piezoelectric transformer is shown in Japanese Publication No. JP 63-272084. In this application, the piezoelectric substrate is H-shaped of which the central portion is polarized vertically and the remaining portions are polarized longitudinally in a single direction. The transformed voltage is output from the two terminals of the H-shaped substrate. The device can comprise more than one output terminals but the H-shaped construction is difficult to be manufactured. Similarly, such a transformer has a low operational frequency and a large amount of noise.